Liquid crystal display devices have been applied to, for example, clocks and watches, calculators, a variety of measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, television sets, clocks, and advertising boards. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, and vertical alignment (VA) and IPS (in-plane switching) types involving use of a TFT (thin film transistor). Liquid crystal compositions used in such liquid crystal display devices need to satisfy the following requirements: being stable to external elements such as moisture, air, heat, and light; having a liquid crystal phase in a wide temperature range mainly including room temperature as much as possible; having a low viscosity; and enabling a low driving voltage. In addition, liquid crystal compositions are composed of several to tens of compounds to adjust, for example, the dielectric anisotropy (Δ∈) and refractive index anisotropy (Δn) to be optimum to individual liquid crystal display devices.
A liquid crystal composition having a negative Δ∈ is used in VA displays and widely applied to, for instance, liquid crystal TVs. In all types of driving, there have been demands for driving at low voltage, a quick response, and a broad range of operation temperature. In other words, Δ∈ with a large absolute value, a low viscosity (η), and a high nematic phase-isotropic liquid phase transition temperature (TNI) have been demanded. In order to determine Δn×d that is a product of Δn and a cell gap (d), the Δn of a liquid crystal composition needs to be adjusted to be in a proper range on the basis of the cell gap. In addition, a quick response is important in liquid crystal display devices applied to television sets or other apparatuses, which generates a need for a liquid crystal composition having a small rotational viscosity (γ1).
In order to improve the viewing angle characteristics of VA displays, MVA (multi-domain vertical alignment) liquid crystal displays have been widely used, in which protrusions formed on a substrate enable liquid crystal molecules in pixels to be oriented in multiple directions. In MVA liquid crystal display devices, viewing angle characteristics are good; however, there is a difference in the response speed of liquid crystal molecules between part adjacent to the protrusions formed on a substrate and part distant therefrom, and the liquid crystal molecules that are distant from the protrusions and thus show slow response speed cause the response speed as a whole to be problematically insufficient, which results in a problem of decreased light transmittance attributed to the protrusions. In order to overcome this problem, PSA liquid crystal display devices (polymer sustained alignment, including PS (polymer stabilized) liquid crystal display devices) having a different structure from general MVA liquid crystal display devices have been developed as a technique for giving an even pretilt angle in segmented pixels without formation of protrusions, which are not light-transmissive, in a cell. In production of PSA liquid crystal display devices, a small amount of a polymerizable compound is added to a liquid crystal composition, the liquid crystal composition is introduced into a liquid crystal cell, and then an active energy ray is radiated thereto under application of voltage between electrodes to polymerize the polymerizable compound contained in the liquid crystal composition. A proper pretilt angle can be therefore given in segmented pixels; as a result, increased light transmittance leads to an enhancement in contrast, and giving an even pretilt angle enables high response speed (e.g., see Patent Literature 1). In PSA liquid crystal display devices, however, the addition of a polymerizable compound to a liquid crystal composition causes a problem that defective display such as image sticking is caused in active-matrix liquid crystal display devices in which a high voltage holding ratio is required.
Meanwhile, an increase in the size of the screens of liquid crystal display devices causes a great change in a method for manufacturing liquid crystal display devices. In particular, since typical vacuum injection techniques require much time to be taken in a process for producing large panels, production by an ODF (one-drop-fill) technique has been becoming popular in production of large panels (e.g., see Patent Literature 2).
This technique enables a reduction in time taken for injection as compared with vacuum injection techniques and has therefore become mainstream in manufacturing of liquid crystal display devices. A new problem, however, has arisen, in which droplet stains made by droplets of a liquid crystal composition remain in liquid crystal display devices in the form of landed droplets even after the liquid crystal devices have been completed. The term “droplet stains” is defined as a phenomenon in which white stains of a dropped liquid crystal composition emerge in a black display mode. In general, the occurrence of droplet stains depends on liquid crystal materials to be used in many cases, and the cause of it has been still unclear.
A technique for reducing droplet stains has been disclosed; in this technique, a polymerizable compound contained in a liquid crystal composition is polymerized to form a polymer layer in a liquid crystal composition layer, so that droplet stains generated by an effect of an alignment film can be reduced (e.g., see Patent Literature 3). Such a technique, however, has a problem of image sticking caused by the polymerizable compound added to the liquid crystal composition as in PSA liquid crystal display devices, and the effect on a reduction in droplet stains is insufficient. Hence, development of a liquid crystal display device which retains basic characteristics inherent in liquid crystal display devices and in which image sticking and droplet stains are less likely to be caused are demanded.